Lipid peroxidation products, such as 4-hydroxy-2-nonenal and malondialdehyde, have previously been shown to cause structural changes to proteins, including introduction of carbonyl groups into amino acid side chains. The level of carbonyl groups has proven to be a valuable measure of oxidative protein damage associated with aging, oxidative stress, and a number of diseases. The ability of unsaturated fatty acids to modify amino acid residues in bovine serum albumin, glutamine synthetase, and insulin in the presence of a metal-catalyzed oxidation system [ascorbate + Fe(III)] is under investigation and has been shown to be dependent on the degree of unsaturation of the fatty acid. The fatty acid-dependent generation of carbonyl groups and loss of lysine residues increased in the order: methyl linoleate less than methyl linolenate less than methyl arachidonate. Lower levels of lipid hydroperoxides were formed in the presence of protein, as compared to that in the absence of protein, when methyl linolenate and arachidonate were oxidized by reactive oxygen species. This suggests that hydroperoxides and/or lipid radicals obtained from peroxidation of polyunsaturated fatty acids react with amino acid residues. Lipid hydroperoxides alone cause only a slight increase in protein carbonyl levels, whereas substantial increases occur when Fe(II) is also present. This indicates that alkoxyl radicals derived from the Fe(II)- dependent cleavage of lipid hydroperoxides are likely responsible for the lipid peroxide-dependent generation of protein carbonyls. These results demonstrate that early events in lipid peroxidation contribute to protein oxidation. - protein oxidation, lysine, lipid peroxidation, lipid hydroperoxides, aldehydes